A large number of printing apparatuses are being produced which are capable of printing an image based on image data captured from digital still cameras, digital video cameras, cellular telephones and the like. One example of a printing method being adopted for such printing apparatuses is a thermal transfer method. In printing apparatuses using this thermal transfer method (hereinafter, called “thermal printers”), multiple heat generating elements are arranged in the main scanning direction of a thermal head and selectively driven to generate heat according to print data. The heat melts the ink of the dye ink portion of the ink sheet to transfer the melted ink to a paper which has a dye receiving layer on its front face, such that the printing of an image of one main scanning direction is performed. Further, by conveying the paper in the sub-scanning direction in synchronism with the printing of an image of one main scanning direction is completed, an image composed of multiple main scanning lines is printed on the paper.
Among thermal printers, sublimation thermal printers print by sublimating the ink of an ink sheet from a solid to a vapor and causing it to affix to a paper. Because this type of thermal printer can control the heat quantity of the thermal head and the number of driving times of the thermal head and thereby modify the density of each pixel of a printed image, it is capable of printing an image with smooth gradients and rich tonality. Because of this, the thermal printers are widely used for photograph printing.
FIGS. 7A and 7B depict views illustrating an ink sheet for use with a general thermal printer. FIG. 7A depicts an ink sheet for color printing and FIG. 7B depicts an ink sheet for monochrome printing.
In the ink sheet depicted in FIG. 7A, each sublimation dye-based color ink portion, yellow (Y) 701, magenta (M) 702, cyan (C) 703, is arranged in frame-sequential order on the upper surface, in the longitudinal direction of the ink sheet. Then, after the color ink portions, an overcoat portion (OP) (transparent resin of a thermo-melting coating material) 704 is provided in order to protect the formed image layer transferred onto a paper. Also, between each ink portion as well as between cyan (C) 703 and the overcoat portion 704, cue markers 705 for cue position detection of each portion is arranged. However, this does not apply to configurations using special purpose ink sheet (FIG. 7B) for black and white image printing. In FIG. 7B, the black sublimation dye-based ink portion 710 and the overcoat portion 711, respectively, are repeatedly arranged in frame-sequential order. Also, a marker 712 for cue position detection of each portion is provided between the black ink sheet portion 710 and the overcoat portion 711. It should be noted that the arrows in FIGS. 7A and 7B indicate the conveyance direction of the ink sheet.
FIG. 8 depicts a view illustrating a cross-section of a paper onto which Y, M and C ink (sublimation dye) is transferred using the ink sheet of FIG. 7A.
In FIG. 8, reference numeral 801 denotes a dye-receiving layer of a paper, reference numeral 802 denotes a transferred yellow (Y) dye sublimation layer, reference numeral 803 denotes a magenta (M) dye sublimation layer, reference numeral 804 denotes a cyan (C) dye sublimation layer and reference numeral 805 denotes a overcoat (OP) layer.
FIG. 9 depicts a view explaining a printing example of an image printed using the ink sheet shown in FIG. 7A.
When using this type of ink sheet to print a single image, processing is done to thermally transfer, as one set, the dye ink portions of the three colors Y, M and C as well as the overcoat portion to a paper. Therefore, the ink sheet stored in the ink sheet cartridge has a plurality of sets, each having color ink portions of Y, M and C and the OP portion (overcoat portion), as many as the number of images that the ink sheet guarantees. Normally, the length (in the longitudinal direction of the ink sheet) of each of the Y, M and C color ink portions as well as the OP portion is set to a length by which it is possible to print an image of the longitude size of a target paper.
For example, on a thermal printer which uses a paper roll as a paper, the length of the sub-scanning direction of an image relative to the main scanning direction, which is in the alignment direction of the heating elements of the thermal head, is optionally set according to a cutting position of the roll paper. Accordingly, as shown in FIG. 9, it is also possible to print a panorama- (wide-) sized image 910 comprising two frames, which use two sets of color ink portions of Y, M, C and OP portions. In FIG. 9, the left frame (first image) 911 of the panoramic image 910 is transferred as resins of the color ink portions 701-703 of Y, M, C and the OP portion 704 for the first frame of the ink sheet and printed on the paper. In the same way, the right frame (second image) 912 is transferred as resins of the color ink portions 701-703 of Y, M, C and the OP portion 704 for the second frame of the ink sheet and printed on the paper. Further, by continuing printing of images for the third and fourth frames, adjacent to the second image 912 of the panoramic image 910, it is possible to print an image having an optional length in the sub-scanning direction (the sheet conveyance direction).
FIG. 10 depicts a cross-sectional view of an edge position of a first frame image on a paper on which the first frame image is printed
In this FIG. 10, reference numeral 1001 denotes a paper having an ink-receiving layer, on which the first image is printed using a sublimation-type heat transfer. Because the OP layer 1005 carries out a protective role, when compared to the color ink layers, the Y, M and C colors (1002 to 1004), it is transferred longer on the paper 1001 by a difference 1006 corresponding to several numbers of dots of image. However, when transferring each frame of neighboring images, as with the panorama-sized image as shown in FIG. 9, the ink sheet is stuck to the paper at the point where the first image and second image come into contact if printing of the first image is carried out using Y, M, and C color ink portions and OP portion. This is because, at the difference 1006 of the OP layer 1005, the Y color ink portion 701, which is first used to transfer the second image, is contact-pressured to transfer the Y color ink on the OP layer 1005. That is, in the portion at which the first and second images come into contact, when the Y color of the second image is transferred on the OP layer 1005, in which the OP portion 1005 was transferred for the first image, and the OP layer 1005 is reheated by the thermal head and melted, causing the Y color ink portion 701 of the ink sheet to stick to the OP layer 1005 due to the melting of the transferred OP layer 1005.
Japanese Patent-Laid-Open No. 2006-315215 describes a method for solving the problem that an ink sheet is stuck to a printing paper if a thermal transfer printing is carried out when a printing sheet for thermal dye sublimation printing is not set. Also, Japanese Patent-Laid-Open No. 2004-082610 discloses a method for printing an image larger than a given size like a panoramic image using a standard ink ribbon. However, neither of these references either discusses or provides a solution to the problems.